1. Technical Field
The present disclosure relates to an abnormality detection device for a continuously variable transmission and a method of detecting abnormality of the continuously variable transmission.
2. Related Art
These days, continuously variable transmissions (CVT), such as a chain-type CVT or a belt-type CVT, which allow a gear ratio to be continuously changed, are widely in practical use. In general, the continuously variable transmission is controlled by an electrical control device using a microcomputer, for example. Although it is extremely rare for such an electrical control device to have a failure, a fault might occur in the microcomputer, for example, a fault in a memory such as a RAM and a register (for example, written data and read data do not match), a fault in a logical operation circuit, and variation in clock signal might occur. In addition, an error in setting or writing ROM data (such as control data) might occur.
Japanese Patent No. 3362637 discloses a technology for a continuously variable transmission that determinants a gear ratio by performing transmission control using a line pressure as source pressure, which accurately determines a failure of a controller (electrical control unit) in the continuously variable transmission and which prevents a sudden deceleration due to a shift of the continuously variable transmission to the lowest speed gear ratio at the time of failure.
With this technology, a sub-controller first divides a primary pulley rotational speed by a secondary pulley rotational speed to determine an actual gear ratio, and retrieves a preset value of line pressure solenoid drive duty based on the actual gear ratio. It is to be noted that the preset value of line pressure solenoid drive duty is a solenoid drive duty which is defined for a gear ratio and corresponding to a maximum pressure of target line pressure value. Therefore, a range of duty exceeding the preset value of line pressure solenoid drive duty is a high line pressure range which is not achievable by nature in the transmission control with the gear ratio.
Next, the sub-controller reads a solenoid drive duty (control command value of line pressure) to a line pressure solenoid from a main controller, and determines whether the main controller is normal or abnormal according to whether or not the solenoid drive duty exceeds the preset value of line pressure solenoid drive duty. When the main controller is abnormal, the sub-controller sets a main controller abnormality determination signal to 0 and fixes the current gear ratio, whereas when the main controller is normal, the sub-controller sets the abnormality determination signal to 1 and performs normal transmission.
As mentioned above, the technology described in Japanese Patent No. 3362637 determines the main controller to be abnormal in the case where the solenoid drive duty (control command value of line pressure) to the line pressure solenoid exceeds the preset value of line pressure solenoid drive duty which is determined based on the actual gear ratio, that is, where the solenoid drive duty is in a high line pressure range which is not achievable in actual line pressure control.
However, even when the target gear ratio for a continuously variable transmission is in a range achievable in normal control and the solenoid drive duty (control command value of line pressure) is in a range of normal control, sudden deceleration may occur depending on a state of the vehicle, such as the output torque of the engine and a running resistance. That is, for example, when transmission is shifted down, acceleration or deceleration may occur depending on conditions such as the output torque of the engine and the running resistance.
However, the technology described in Japanese Patent No. 3362637 may not detect an abnormal state in which although a target gear ratio is in a range achievable in normal control, sudden deceleration occurs depending on conditions such as the output torque of the engine and the running resistance.